The measurement of blood flow is an essential component for physiological control of a ventricle assist device, to assist the heart in functioning. Such devices are well known, and, as indicated for example in U.S. Pat. Nos. 6,149,683; 6,158,984; 6,234,772; and 6,368,083 may comprise a blood pump having a rapidly rotating, electrically powered impeller for pumping of blood, generally in a supplemental manner, to assist a failing heart. However, too much flow through the ventricle assist device (VAD) can lead to ventricle collapse and damage to the myocardium. Too little flow can result in the VAD not producing proper therapeutic support for the patient. Thus, the blood flow provided by the VAD must be closely monitored at all times.
For a VAD utilizing a rotary pump, the only parameters which are available for flow estimation, in the absence of supplemental sensors, are the rotational speed of the impeller and the power or the electrical current passing through the electric motor. Such sensorless flow estimation can be determined through sensing of the current or the back EMF of the system.
Rotary pumps of the type described above have a characteristic performance curve, such as the pressure head-flow curve, which relates the pump head and the blood flow rate provided by the pump at a given rotational speed for the impeller. “Pump head” is related to the pressure increase created by the pump, and is a known term. Such a standard curve is used to help select the appropriate pump for a given system application. However, a problem arises in the obtaining of a reliable flow estimate for a particular clinical situation, using the standard performance curves for a particular pump. This arises from the fact that the viscosity of a patient's blood is variable to an extent, a sufficient extent to render estimated pump flow rates inaccurate unless the current blood viscosity is known. The patient's health, gender, degree of hydration, hematocrit and certain medications can affect blood viscosity to a degree that is sufficient to throw off estimates of pump flow based upon the pump power and/or rotational speed. Thus, a periodic measurement of blood viscosity is needed in order to maintain the accuracy of VAD flow and pressure head estimation.
Thus, without careful and continuous monitoring of the viscosity of the blood of the patient, one cannot obtain an accurate estimate of the flow rate of the blood pumped by the VAD. This, of course, renders difficult or impossible an estimate on how the native heart is performing, and other clinical data becomes effectively unavailable from the data that can be obtained from the ventricle assist pump device itself. Thus, it becomes necessary to utilize differential pressure sensors and the like to obtain better data. Without such sensors, much accurate clinical data pertaining to the patient has been unavailable from an analysis of data from the VAD pump.